1. Field of the Invention
This present application relates to lens imaging, and more particularly, to a system for lens imaging that mitigates the effects of lens heating. The systems and methods described herein mitigate the heat signature without substantially affecting the imaging performance.
2. Description of the Prior Art
Lithographic projection is a technique whereby light (usually from a laser) is projected through a photo mask into a pupil lens, and then projected via the lens onto a wafer. This technique is used, for example, to print integrated circuits (ICs). The photo mask and the specific illumination mode used (for example, a dipole illumination mode) mean that certain parts of the lens may absorb more heat than others. Over time, the effect of the heating may increase. The net result is a non-uniform temperature distribution across the lens. Wavefront aberrations and lens distortion may occur as a result. When the shape of the lens is distorted, the projected light onto the wafer may be influenced. Therefore, lens heating can significantly affect the quality of imaging.
When determining how temperature affects a lens, certain factors may be taken into consideration. Certain areas of the lens may be more sensitive to heat than others, and therefore the aberrations may be greater. In addition, it might not be sufficient to only consider the effect of heating on the pupil (determined by the numerical aperture, NA). Although light outside the pupil might not be projected onto the wafer, it might still contribute to the lens heating, by causing wavefront aberrations.
One method of characterizing how temperature affects a lens is by using a set of Zernike polynomials. Zernike polynomials are linearly independent terms that describe, up to the nth order, aberrations across the lens. If the lens heating effect is known, designers can compensate by adjusting various optical elements to substantially cancel out the effects; however, it is not known how to fully correct for these aberrations. Correcting for one Zernike term may induce other aberrations. Also, some correction methods are limited to certain ranges.